The invention relates to a method of transmitting or storing, over an interference affected channel, digital audio signals, wherein transmission errors are detected at the receiving end and corrected if necessary or masked, with the masking being effected in that the interfered-with signal section is muted or replaced by a signal preceding the interfered-with signal section in the same channel or by a synchronous, not interfered-with signal section of an adjacent channel. Such a method is disclosed in German Patent DE 3,638,922.C2.
In digital audio signal transmissions and storage, the received and read-out audio signals, if they contain bit errors, may either be not decodable at all or not decodable to in part after all error detection and error correction methods have been exhausted. In that case, it is the custom to switch to a decoder for muting over a broad band in such a way that for a certain time period the entire signal is set at 0. In the case of digital audio signals transmitted by radio, this case occurs relatively frequently at the fringes of the reception area which is extremely annoying particularly in connection with mobile reception. The same applies for audio signal storage if the tape material or audio heads are worn to a degree that exceeds a tolerance value.
To reduce the annoying effect of missing signals in radio transmissions, German Patent DE 3,638,922.C2 discloses a mutual offset in time between the left and right stereo channels and, if there is an uncorrectable signal interference, placing the complementary stereo information transmitted at an earlier or later point in time instead of the interfered-with original information. Although the complementary stereo information is not identical with the associated, interfered-with original information, but is connected with it only by way of left-right correlation, such a substitution is acceptable in any case for a short period of time since direction and distance perception as well as the perception of spatial relationships are subject to a certain inertia in the human ear. However, if such a masking technique is employed for a longer period of time, the stereophonic impression is lost, since the masking always covers the full bandwidth of the interfered-with signal and therefore also replaces spectral signal components that did not suffer interference.
In contrast thereto, it is the object of the invention to provide, in a method of the above-mentioned type, a subjectively better masking of errors which permits better reconstruction of the interfered-with original audio event and, particularly also during longer use, maintains the stereophonic impression.
The above and other objects are accomplished according to the invention by the provision of a method of transmitting or storing, over an interference affected channel, digital audio signals that have been subjected to data reduction to result in a reduced data, digital audio signal that is present in at least one of the time domain and spectral domain, depending on source coding, the method comprising the steps of: detecting an interfered-with signal section in the reduced data, digital audio signal at a receiving end; and masking the interfered-with reduced data, digital audio signal section by one of (a) muting only interfered-with spectral values or subbands, or groups of spectral values or subbands in the signal section and (b) replacing only interfered-with spectral values or subbands, or groups of spectral values or subbands on the signal section with a signal component from the same channel or an adjacent channel including at least one of time domain sampled values, spectral domain sampled values, scale factors, and control information, depending on the source coding of the reduced data, digital audio signal.
The invention is based on the consideration of intentionally muting, repeating, estimating or replacing for a certain period of time only those spectral components of a complete audio signal that have actually experienced interference. The invention takes advantage of the fact that reduced data, digital audio signals (i.e., digital audio signals subjected to known data reduction processes) are present in the time and spectral domains, depending on whether they are subdivided into subbands (subband coding) or into spectral values (transformation coding). The following masking strategies can be employed:
1. Muting
With this masking strategy, only those spectral components of the interfered-with signal are muted which have actually been interfered with. Depending on the source coding method employed, these may be individual subbands or spectral values or groups of subbands or spectral values of a digital audio signal that has been subdivided into n subbands or spectral values, with n being a value equal to or greater than 1.
2. Repeating
With this masking strategy, components of the same channel signal are employed as substitutes for the interfered-with signal components, which hereinafter will be called xe2x80x9crepeatingxe2x80x9d. Depending on the source coding method employed, the components to be repeated may be individual subbands or spectral values or groups of subbands or spectral values of a digital audio signal that has been subdivided into n subbands or spectral values, with n again being a value equal to or greater than 1. Again depending on the source coding method employed, these components may be composed of synchronous or spectral sampled values as well as control informations or scale factors. The repetition may also be effected several times.
3. Left-Right Substitution
In this case, synchronous components of the audio signal that are present without interference or in processed form in the adjacent channel are employed as substitutes, which will hereinafter be called xe2x80x9cleft-right substitutionxe2x80x9d. The components to be replaced may be, as in the case of repeating and depending on the source coding method employed, individual subbands or spectral values or groups of subbands or spectral values of an audio signal that has been subdivided into n subbands or spectral values, with n again being a value equal to or greater than 1. Again depending on the source coding method employed, these components may be composed of time domain or spectral domain sampled values as well as of control information or scale factors.
4. Estimating
With this masking strategy, not interfered-with or processed components of the same channel signal or of the signal from the adjacent channel are utilized by way of estimation (e.g. interpolation) to determine the components required to substitute interfered-with components. Due to the probability of combinations in the time domain or the spectral domain, a conclusion can be drawn from the use of non-interfered-with or processed spectrally or timely adjacent components of the same channel signal or of a signal from the adjacent channel as to the original content of interfered-with components that must be replaced. Depending on the source coding method employed, the components to be estimated may be individual subbands or spectral values or groups of subbands or spectral values of an audio signal that has been subdivided into n subbands or spectral values, with n here again being a value equal to or greater than 1. Again, depending on the source coding method employed, these components may be composed of time domain or spectral domain sampled values as well as of control informations or scale factors. Various methods for fading the signals in and/or out can be employed for the above-mentioned masking strategies. That means that the changeover from the not interfered-with component to the replaced or muted component and/or from the replaced or muted component to the not interfered-with component is made over a certain transition time and with a certain transition function so that sudden transitions are avoided.
All of the above-mentioned masking strategies can be combined with one another if required.
In connection with the described masking strategies, the invention takes advantage of certain characteristics of the human sense of hearing. In the case of muting, repeating or estimating, the auditory characteristics with respect to timely and/or simultaneous masking thresholds are utilized to the extent that these masking measures remain substantially inaudible or hidden as long as they do not exceed a certain time, spectrum and level range.
In the case of the left-right substitution, advantage is taken of the fact that direction and distance perception as well as the perception of spatial relationships are subject to a certain inertia. This can be utilized to the extent that short-term changes in the display of directions and distances of sound sources and of a spatial impression remain inaudible if they are shorter than a certain length of time and do not occur too frequently. If these two conditions can be met, the correct application of the described masking strategies results in a substantial improvement over conventional, known masking techniques.